Synesthesia

Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae or synaesthesiae), from the ancient Greek σύν (syn), "together," and αἴσθησις (aisthēsis), "sensation," is a neurologically based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes.

http://en.wikipedia.org/wiki/Synesthesia